This specification describes, with references to the accompanying drawings, an improvement in a preamplifier circuit for a microphone component of a hearing aid.
Generally speaking, a hearing aid utilizes three basic components, a microphone, an amplifier and a receiver or loudspeaker component. Briefly, the microphone generally includes a transducer for converting incoming sound pressure levels to corresponding electrical signals. The amplifier amplifies these electrical signals to a desired level, and the receiver or loudspeaker component translates these amplified electrical signals to an acoustical sound output, or outgoing sound pressure level.
Generally, the microphone component includes a transducer such as an electret microphone, and a buffer amplifier or xe2x80x9cpreamplifier.xe2x80x9d Often, a CMOS amplifier component is used as the preamplifier. The input of this CMOS preamplifier must be biased to ground level. However, since the signal source in the electret microphone is typically a relatively small capacitor, the impedance of the preamplifier input should be relatively large, and preferably, as large as possible. Currently, bias resistor values of about 10 Gohms are utilized. For further noise reduction, this value might be increased up to 100 Gohms. However, these resistors cannot be implemented on ICs but must be made on a thick film hybrid. As an alternative to the resistors for the preamplifier input impedance, a pair of small diodes coupled in parallel but in opposite polarity are sometimes used. This permits a relatively large impedance value to be obtained, which value is strongly dependant upon process variations. However, the use of diodes for this purpose has the disadvantages of slow settling and slow overload recovery, which are characteristics of diodes.
Accordingly, it is an object of the invention to provide a high input impedance for a CMOS preamplifier which overcomes the above-noted shortcomings of utilizing bias resistors or reverse-polarized parallel diodes.
In accordance with the foregoing, a high input impedance circuit for a buffer amplifier/preamplifier of a hearing aid microphone comprises a transistor coupled across an input of said buffer amplifier/preamplifier and providing a high impedance transconductance, and a control circuit operatively coupled with the transistor to control the effective value of the input impedance provided thereby.